Rocky planetary bodies aren't uniform: different parts of them have different densities due to variations in composition. Mountains composed of granite will not be the same density as ones built of basalt, and both will be more dense than an ice sheet.

This variation is reflected in the gravitational pull exerted by these mountains, though you'd never notice the difference. The difference in the thickness of tectonic plates produce a variation in Earth's gravitational field, for example, but Earth-orbiting satellites typically don't need to take that into account.

However, measuring these fluctuations can reveal a lot about the interior of a rocky world. The GRAIL mission (Gravity Recovery And Interior Laboratory), launched in 2011, is designed to map the details of the Moon's composition with an eye toward understanding both its interior and its history. GRAIL's design is based on an earlier Earth-orbiting mission known as GRACE (Gravity Recovery And Climate Experiment), a collaboration between the United States and Germany in 2002.

Both GRAIL and GRACE consist of two satellites orbiting in tandem. Each uses a microwave (K-band) antenna to measure the distance to the other. The relative position of the satellites can map the fluctuations in the gravitational field to higher precision than a single instrument would be able to do.

Each GRAIL spaceship is about 200kg in mass and the size of a refrigerator. The craft follow a nearly polar orbit, looping around the Moon such that, as it rotates on its axis, GRAIL was able to take measurements of the entire body in swaths (akin to the segments of an orange).

As the two satellites—known as GRAIL-A ("Ebb") and GRAIL-B ("Flow")—fly over the Moon's surface, they speed up or slow down relative to each other, as variations in the density of the lunar interior exert different gravitational pulls. The ranging instruments for measuring relative position pinpoint the distance between the craft to a matter of microns: a few millionths of a meter. Even slight differences in density due to lunar crust composition or interior variation can be measured this way.

GRACE is used to determine how water moves and is stored in the earth. It's not just like 'hey there's some heavy stuff there let's dig it up.' Although that would be a possibility on the moon, should anything useful be found there... *cough*unobtanium*cough*

I'd be interested in some more detail about how the gravity differences change the orbital dynamics, and (more importantly) how the math works on converting the spacecraft distance measurements into gravity maps.

I'd be interested in some more detail about how the gravity differences change the orbital dynamics, and (more importantly) how the math works on converting the spacecraft distance measurements into gravity maps.

Basically, assume the motion of the spacecraft follows known laws with unknown constants, if you get enough measurements of the motion then you can solve for the constants. Look up a Kalman filter to see a practical method for complex equations of motion.

GRACE is used to determine how water moves and is stored in the earth. It's not just like 'hey there's some heavy stuff there let's dig it up.' Although that would be a possibility on the moon, should anything useful be found there... *cough*unobtanium*cough*

GRACE is used to determine how water moves and is stored in the earth. It's not just like 'hey there's some heavy stuff there let's dig it up.' Although that would be a possibility on the moon, should anything useful be found there... *cough*unobtanium*cough*

Sounds like this would be more interesting to do for the earth so we could dig up the places with curious results and see what's going on.

They did mention that we've been doing this for the earth since 2002 with GRACE.

And that GRACE, due to Earth atmosphere has to orbit ~ 500 km up instead of GRAIL ~ 50 (in the extended close-range mission). So GRAIL will give the best "ground truth" reference, as it were.

But the real reason is to understand the Moon better, since it will make us understand especially the Earth better. (Having the same composition, roughly same history, being differentiated so telling on how a core works, et cetera.)